Interpretive Summary: The ability of bacteria that are harmful to humans to survive various fresh produce wash treatments plays a crucial role in foodborne diseases. Acidic produce, such as apple cider and juice, as well as fresh-cut melons and tomatoes have been implicated in recent outbreaks of infections caused by E. coli O157:H7. The ability of bacteria to survive is influenced by the environmental conditions they were subjected to before and during handling and preparation. In this study, we show that pathogenic E. coli strains are able to alter their genetic regulatory mechanisms to survive environmental stress conditions. Understanding how pathogens overcome various stress conditions during food processing and handling will advance our knowledge of how human pathogens enter our food chain. The research will benefit the fresh produce industry, as well as increasing the microbial food safety of the American food supply.

Technical Abstract:
Stationary phase sigma-factor (RpoS) regulates many of the cell’s responses towards environmental stress conditions such as heat, acid and alkali-shocks. On the other hand, mutations at the rpoS locus have frequently been detected among pathogenic as well as commensal strains of Escherichia coli. The objective of this study was to perform a functional analysis of enterohemorrhagic E. coli strains from food-borne outbreaks with reference to their RpoS-mediated stress responses. E. coli strains belonging to serotypes O157:H7, O111:H12 and O26:H11 exhibited polymorphism for the two phenotypes widely used to monitor rpoS mutations: heat-tolerance and glycogen synthesis, even though they synthesized the oxidative acid-resistance system by the rpoS-dependent pathway. Both of the phenotypes were restored upon introduction of a wild-type rpoS on a low-copy number plasmid. During the transition from a mildly-acidic growth condition to alkaline stress, the cells relied heavily on rpoS functionality. Some strains were able to overcome negative-regulation by the RpoS and induced higher Beta-galactosidase activity without compromising their acid-resistance. There were no major differences in the DNA sequences in the rpoS coding regions among the various strains. The heterogeneity of rpoS-dependent phenotypes observed for stress-related phenotypes was not evident in the Caco-2 cells adherence assays. These results show that some pathogenic E. coli strains can modulate their rpoS-dependent cellular responses without affecting their acid-tolerance capability. Such adaptation processes may have significant impact on a pathogen’s survival in food processing environments as well in the host’s stomach and intestine.